Films with reduced permeability to vapors such as water, oxygen, and carbon dioxide are useful for a variety of applications, one of which is to package foods. Such films are typically composites of materials. For example, one layer is often a flexible polymer, such as a polyethylene or polypropylene, while another layer is coated on or coextruded with the one layer and serves as a barrier layer. Barrier layers can generally be viewed as substantially organic based or substantially inorganic based.
Examples of organic barrier layers are described in a number of U.S. Patents. For example, U.S. Pat. No. 4,792,488, issued Dec. 20, 1988, inventor Schirmer, describes a coextruded multilayer film said to have high oxygen barrier properties and composed of saran (e.g., polyvinylidene chloride copolymers or PVDC). U.S. Pat. No. 4,894,291, issued Jan. 16, 1990, inventor Ofstein, describes oxygen barrier materials in laminates that include nylon, PVDC, and polyethylene terephthalate (PET) and ethylene vinyl alcohol copolymers. U.S. Pat. No. 4,888,249, issued Dec. 19, 1989, inventors Flores et al., describes composites or multi-layers of saran polymers with polyamides or high density polyethylene to assist in oxygen, water, and aroma barrier properties. However, such organic composites pose problems in recycling since they often cannot be simply melted and reformed and they can pose flavor or taste change problems.
U.S. Pat. No. 4,422,915, issued Dec. 27, 1983, inventor Wielonski, describes coatings that are substantially polymeric and consist essentially of a plasma formed polymer matrix, but with particulates distributed in the cross-linked polymer, such as aluminum, boron, nickel, and the like metals and metalloid elements, in order to alter the color of the polymer and to provide selected visual colors. Applications for these colored polymeric coatings are suggested as coating sheet steel for corrosion protection, encapsulating coatings for microelectric circuits and serving as decorative and/or protective coatings for metal, paper, glass, cloth, and plastic materials and articles. The cross-linked polymer is formed by flowing a plasma-polymerizable material (such as hexamethyldisiloxane) into a vacuum chamber while concurrently imposing an r.f. current through a cathode which includes a substrate affixed thereto. An electron beam is struck and produces the particulates in the deposited plasma-form polymer by evaporating the appropriate metal. However, these substantially organic films are not necessarily inert where one would wish inertness due to the desired application.
Inorganic barrier coatings are known and typically are advantageously inert. These inorganic coatings can be produced as thin layers in vacuum deposition chambers. For example, U.S. Pat. No. 3,442,686, issued May 6, 1969, inventor Jones, describes a packaging film composite in which silicon oxide coatings in the range of 0.2 to 2 microns are produced by electron beam evaporation of silicon monoxide in a vacuum chamber. The patentee describes these inorganic coatings as substantially continuous. More recently, U.S. Pat. No. 4,702,963, issued Oct. 27, 1987, inventors Phillips et al., describes flexible polymer packaging films having thin films of inorganic coatings. The inorganic coatings are silicon dioxide or monoxide in which at least chromium, tantalum, nickel, molybdenum, or oxides of these materials are co-deposited and are said to serve as an adhesion layer and assist in lower gas and vapor permeability. Layers are suggested to be produced in a roll coating machine with vacuum deposition chambers.